A data communication system employing an IC card has been proposed for applications to ski lifts, automatic card examination devices for railways, automatic sorting of luggage or the like.
FIG. 1 shows a structure of a communication system employing a non contact type IC card as an example of data communication systems employing IC cards. This system includes an interrogator 240 (mounted on a gate of a ski lift, for example) and a non contact type IC card 220.
Interrogator 240 sends a high frequency carrier from an oscillating circuit 249 through an antenna 241 under the control of a control portion 248 on the side of the interrogator. When non contact type IC card 220 comes to the vicinity of interrogator 240, non contact type IC card 220 receives the high frequency carrier at an antenna 223 thereof. A power supply generation circuit 225 converts the received high frequency wave into a d.c. current and supplies power to other circuit portions. Thus non contact type IC card 220 becomes operable in the vicinity of interrogator 240.
Here, information is transmitted from interrogator 240 to non contact type IC card 220 as the above mentioned high frequency carrier is demodulated at a modulating/demodulating circuit 233. A control portion 235 on the side of the card performs required processes such as rewriting of contents of a memory 237 and answering the received information based on the demodulated information.
On the other hand, the information is also transmitted from non contact type IC card 220 to interrogator 240. As non contact type IC card 220 is not provided with an oscillating circuit, the information transmission is performed as described below. First, unmodulated high frequency carrier is sent from the side of interrogator 240, and an impedance of a resonance circuit 222 is changed by modulating/demodulating circuit 233 at the side of non contact type IC card 220. This change in impedance is detected by interrogator 240 as the impedance change in a resonance circuit 242 on the side thereof and is demodulated by a modulating/demodulating circuit 246. Control portion 248 receives the demodulated information and performs required processes.
When non contact type IC card 220 leaves interrogator 240, the power supply is stopped and therefore the operation of non contact type IC card 220 is stopped. At this time, memory 237, which is a non-volatile memory, holds the stored information regardless of a cease of power supply.
Non contact type IC card 220 as described above can be used as a prepaid card by storing a predetermined count in memory 237 and rewriting data in memory 237 according to a use count.
The communication data between the interrogator and the IC card is enciphered. By this encipherment, an unauthorized computer connected in place of an IC card is prevented from pretending as an authorized IC card (hereinafter this is called “pretense”).
The conventional communication system employing the IC card as described above, however, is not immune to the problem. For example, even with the encipherment of the communication data as described above, the pretense is possible once a cipher is decrypted.
In particular, when the IC card is used for a telephone, once an authentication process to set up the communication is deciphered and a connected state is established, the connected state can be maintained for hours at will.